1. Field of Invention
The present invention relates to a load driving apparatus for driving a load, and more particularly, to a load driving apparatus for driving a load, such as LEDs (light emitting diodes), connected to a power source circuit with a constant current.
2. Description of Related Art
In recent years, LEDs for the backlights of LCDs (liquid crystal displays) are being widespread as one of uses of LEDs. Generally speaking, when LEDs are used for the backlight of an LCD, a predetermined current is passed through multiple LEDs connected in series, whereby light can be emitted. At this time, the number of the LEDs and the amount of the current are determined depending on the amount of light required. Furthermore, a drive voltage for driving the LEDs is generated using a voltage conversion circuit for converting a power source voltage into a predetermined voltage. This voltage conversion circuit detects and feeds back the voltage value or the current value of a predetermined terminal of the LEDs serving as a load, thereby controlling the drive voltage. Such an LED driving technology as described above has been disclosed in U.S. Patent Application Publication No. 2005/0007085 A1 (corresponding to Japanese Laid-open Patent Publication No. 2005-033853), for example.
The load driving apparatus disclosed in the U.S. Patent Application Publication will be described below briefly referring to FIG. 5. By the use of a constant current source J1 connected in series with an external load 110 formed of LEDs, this conventional load driving apparatus drives the external load 110. The constant current source J1 supplies a load current Jo, and a switching power source circuit 100 detects the connection point voltage Vdet1 of the connection point P200 between the external load 110 and the constant current source J1. The switching power source circuit 100 is controlled so that the connection point voltage Vdet1 becomes equal to a reference voltage Vref1 supplied from a reference voltage source B1. At this time, the connection point voltage Vdet1 is required to be set to a predetermined value or more so that the constant current source J1 can sufficiently supply the load current Jo to the load 110. In other words, the larger the load current Jo required for driving the external load 110, the larger the connection point voltage Vdet1. The reference voltage Vref1 is set so that the connection point voltage Vdet1 satisfies this kind of condition.
In the conventional configuration, the reference voltage Vref1 is constant at all times as described above. When the reference voltage Vref1 is determined, the usage range of the load current Jo is assumed to be set to a certain range, and the reference voltage Vref1 is required to be set so that the maximum load current Jo within the range can be passed. However, in the case that an attempt is made to drive the load 110 by passing a small load current Jo, if the reference voltage Vref1 has been set to a relatively large value, a power obtained as the product of the margin of the connection point voltage Vdet1 (that is, the voltage between both terminals of the load current generating section) and the load current Jo is lost as a loss in the constant current source (also referred to as a load current generating section) J1. In other words, the conventional configuration described above has a problem of lowering efficiency when the load is light.